VSB transmission system

ABSTRACT

A vestigial sideband (VSB) modulation transmission system and, a method for encoding an input signal in the system are disclosed. According to the present invention, the VSB transmission system includes a convolutional encoder for encoding an input signal, a trellis-coded modulation (TCM) encoder for encoding the convolutionally encoded signal, and a signal mapper mapping the trellis-coded signal to generate a corresponding output signal. Different types of the convolutional encoders are explored, and the experimental results showing the performances of the VSB systems incorporating each type of encoders reveals that a reliable data transmission can be achieved even at a lower input signal to noise ratio when a convolutional encoder is used as an error-correcting encoder in a VSB system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/968,083 filed Oct. 1,2001, which application is hereby incorporated by reference in itsentirety. Under 35 U.S.C. § 119, this application claims priority toKorean Application Serial No. 2000-57823 filed on Oct. 2, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital communication system, andmore particularly, to a vestigial sideband (VSB) modulation transmissionsystem including a TCM (Trellis-Coded Modulation) encoder and anadditional ½ rate convolutional encoder having a superior statetransition property when connected to the TCM encoder in the system.

2. Background of the Related Art

The TCM coded 8-VSB modulation transmission system has been selected asa standard in 1995 for the U.S. digital terrestrial televisionbroadcasting, and the actual broadcasting incorporating the system hasstarted since the second half of the year 1998.

In general, a digital communication system performs error correctingprocesses to correct the errors occurred at the communication channels.The total amount of the transmitting data is increased by such errorcorrecting coding processes since it creates additional redundancy bitsadded to the information bits. Therefore, the required bandwidth isusually increased when using an identical modulation technique.Trellis-coded modulation (TCM) combines multilevel modulation and codingto achieve coding gain without bandwidth expansion. Also an improvedsignal to noise ratio can be achieved by using the trellis-codedmodulation (TCM) technique.

FIG. 1A and FIG. 1B illustrate a typical TCM encoder used in a typicalATSC 8-VSB system and corresponding set partitions used by the TCMencoder. According to the FIG. 1A, an input bit d₀ is output as c₁ andc₀ after trellis-coded modulation, and then a subset is selected among(−7,1), (−5,3) (−3,5), and (−1,7). Thereafter, an input bit d₁ selects asignal within the selected subset. In other words, when d₁ and d₀ areinputted, one of eight signals (−7,−5,−3,−1,1,3,5,7) is selected by c2,c1, and c0 generated by the TCM encoder. d1 and d0 are called an uncodedbit and a coded bit, respectively.

FIG. 1B illustrates the set partitions used by the TCM encoder used inthe ATSC 8-VSB system. Eight signal levels are divided into foursubsets, each of which including two signal levels. Two signals areassigned to each subset such that the signal levels of each subset areas far as possible from each other as shown in FIG. 1B.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a VSB transmissionsystem and a method for encoding an input signal in the VSB transmissionsystem that substantially obviates one or more problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide a VSB transmissionsystem that can transmit data reliably even at a lower signal to noiseratio and can have an optimal state transition property when connectedto the TCM encoder by using a ½ rate convolutional encoder as anadditional error correcting encoder in the system.

Another object of the present invention is to provide a method forencoding an input signal in a VSB modulation transmission systemenabling a data sender to achieve more reliable data transmission at alower signal to noise ratio and to have an optimal state transitionproperty of a ½ convolutional encoder, which is concatenated to the TCMencoder for error correcting in the system.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, avestigial sideband (VSB) modulation transmission system includes aconvolutional encoder encoding an input signal; a trellis-codedmodulation (TCM) encoder encoding the convolutionally encoded inputsignal; and a signal mapper mapping the trellis-coded input signal togenerate a corresponding output signal.

In another aspect of the present invention, a vestigial sideband (VSB)modulation transmission system includes a ½ rate convolutional encoderencoding an input signal to generate first and second output signals; a⅔ rate trellis-coded modulation (TCM) encoder encoding the first andsecond output signals to generate third, forth and fifth output signals;and a signal mapper mapping the third, forth, and fifth output signals.

There are three different types of ½ rate convolutional encoders thatcan be used in this aspect of the present invention. The first typeincludes a plurality of multipliers, each i th multiplier multiplyingthe input signal by a constant k_(i) to generate an i th multipliervalue; a plurality of memories, a first memory storing the previoussecond output value as a first memory value and each i+1 th memorystoring an i+1 th memory value obtained by adding an i th memory valuestored in a i th memory and the i th multiplier value; and a pluralityof adders, each i th adder adding the i th memory value and the i thmultiplier value, where i=1, 2, 3, . . . , n, and a n+1 th memory valuestored in a n+1 th memory is the second output signal.

The second type of the ½ rate convolutional encoder includes a firstmemory storing the input signal as a first memory value; a second memorystoring the first memory value as a second memory value; a first adderadding the input signal and the second memory value to generate thefirst output signal; and a second adder adding the input signal and thefirst and second memory values to generate the second output signal.

Finally, the third type of the ½ rate convolutional encoder includes afirst memory storing the previous second output value as a first memoryvalue; an adder adding the input signal and the first memory value; anda second memory storing a result from the adder as a second memoryvalue, the second memory value being the second output signal.

In another aspect of the present invention, a method for encoding aninput signal in a vestigial sideband (VSB) modulation transmissionsystem includes the steps of encoding the input signal by theconvolutional encoder; encoding the convolutionally encoded input signalby the TCM encoder; and generating a final output signal my mapping thetrellis-coded input signal.

In a further aspect of the present invention, a method for encoding aninput signal in a vestigial sideband (VSB) modulation transmissionsystem includes the steps of generating first and second output signalsby encoding the input signal using the ½ convolutional encoder;generating a third, forth, and fifth output signals by encoding thefirst and second output signals using the ⅔ rate TCM encoder; andgenerating a final output signal by mapping the third, forth, and fifthoutput signals.

The second output signal can be generated using three different methodsin the last aspect of the present invention described above. The firstmethod for generating the second output signal includes the steps ofmultiplying the input signal by a constant k_(i) to generate an i thmultiplier value for i=1, 2, 3 . . . n ; storing the previous secondoutput value as a first memory value; and storing an i+1 th memory valueobtained by adding an i th memory value and the i th multiplier valuefor i=1, 2, 3 . . . n, where the second output signal is an n+1 thmemory value.

The second method for generating the second output signal includes thesteps of storing the input signal as a first memory value; storing thefirst memory value as a second memory value; generating the first outputsignal by adding the input signal and the second memory value; andgenerating the second output signal by adding the input signal and thefirst and second memory values.

Finally, the third method for generating the second output signalincludes the steps of storing the previous second output value as afirst memory value; adding the input signal and the first memory value;storing the value resulted from the adding step as a second memoryvalue; and outputting the second memory value as the second outputsignal.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings;

FIG. 1A illustrates a typical trellis-coded modulation (TCM) encoderused in a ATSC 8VSB transmission system according to the related art;

FIG. 1B illustrates set partitions used by a typical TCM encoder of aATSC 8VSB transmission system according to the related art;

FIG. 2 illustrates an error correcting encoder concatenated to a ⅔ rateTCM encoder in a ATSC 8-VSB transmission system according to the presentinvention;

FIG. 3A illustrates a ½ rate convolutional encoder concatenated to a ⅔TCM encoder to be used as an error correcting encoder in a ATSC 8-VSBtransmission system according to the present invention;

FIG. 3B illustrates ⅔ and ⅓ rate convolutional encoders used as an errorcorrecting encoder in a ATSC 8-VSB transmission system according to thepresent invention;

FIG. 4 illustrates a first type of a ½ rate convolutional encoderconcatenated to a ⅔ TCM encoder in a ATSC 8-VSB transmission systemaccording to the present invention;

FIG. 5A illustrates a second type of a ½ rate convolutional encoder usedin a ATSC 8-VSB transmission system according to the present inventionand its corresponding state transition diagram;

FIG. 5B illustrates a third type of ½ rate convolutional encoder used ina ATSC 8-VSB system according to the present invention and itscorresponding state transition diagram;

FIG. 6 illustrates a VSB receiving system corresponding to a ATSC 8-VSBtransmission system according to the present invention;

FIG. 7A illustrates Euclidean distances of a set of output signalsgenerated from the ½ rate convolutional encoder shown in FIG. 5A;

FIG. 7B illustrates Euclidean distances of a set of output signalsgenerated from the ½ rate convolutional encoder shown in FIG. 5B; and

FIG. 8 illustrates performances of ATSC 8-VSB transmission systems wheneach of the ½ rate convolutional encoders shown in FIG. 5A and FIG. 5Bis used.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIG. 2 illustrates a VSB transmission system in which an errorcorrecting encoder is concatenated to a ⅔ rate TCM encoder according tothe present invention. By adding an additional error correcting encoderto the ⅔ rate TCM encoder in the VSB system, it is possible to achieve areliable data transmission even at a lower signal to noise ratio thanthat of the conventional ATSC TCM coded 8VSB system. In the presentinvention, a ½ rate convolutional encoder is used for the additionalerror correcting encoder. In addition, a multiplexer located between theerror correcting encoder and the ⅔ rate TCM encoder classifies the datareceived from each of the error correcting encoder and a ATSC encoderand inputs each data to the TCM encoder. The additional error-correcteddata will be regarded as an error by the ATSC receiver and will bediscarded.

FIG. 3A and 3B illustrate a ½ rate encoder used as an additional errorcorrecting encoder shown in FIG. 2. According to FIG. 3A, an input bit uis processed in the ½ rate encoder to generate two output bits d₁ andd₀, and these are inputted to a ⅔ rate TCN encoder. In FIG. 3B, each of⅔ and ⅓ rate encoders is connected to a ⅔ rate TCM encoder. Since thebit error rate of uncoded bits u₁ is lower than that of a coded bit u₀,the encoder having a higher code rate is used for u₁, and the otherencoder is used for u₀. This will compensate the difference between twoinput bits u₀ and u₁. In addition, the ⅔ and ⅓ rate encoders can beconsidered as being a ½ rate encoder since it has three input bits andsix output bits. Thus, combining encoders having different code ratescan reduce the bit error rate of the whole system. As a result, theadditional encoder can be any one of the ½ rate encoder and thecombination of the ⅔ rate encoder and the ⅓ rate encoder shown in FIG.3A and FIG. 3B, respectively. By adding the additional encoder, theperformance of the system can be enhanced, and this will be shown laterin this section. Considering the signal mapping of the TCM encoder, theerror correcting encoder must be designed so that it has the optimalstate transition property when connected to the TCM encoder.

FIG. 4 illustrates a first type of a ½ rate convolutional encoderconcatenated to a ⅔ TCM encoder in a VSB transmission system accordingto the present invention. The ½ rate convolutional encoder receives aninput bit u and generates a first output bit d₁ by bypassing u. A secondoutput bit d₀ is the value of the N+1 th memory m_(i+1). The ½ rateconvolutional encoder includes N mutipliers, N adders, and N+1 memories.The first memory m₁ stores a previous second output value, the firstmultiplier g₁ multiplies the input bit u by a first constant k₁, and thefirst adder adds the outputs from g₁ and m₁. Similarly, each i+1 thmemory m_(i+1) stores the output from the ith adder, the i th multiplierg_(i) multiplies the input bit u by an i th constant k_(i), and the i thadder adds the outputs from g_(i) and m_(i), where i=2, 3, 4, . . . , N.Finally, the N+1 th memory m₁₊₁ stores the output from the N th adder.Then the value stored in m_(i+1) is output as a second output bit(current). In addition, the second output bit (current) is feedback tothe first memory m₁ for calculating a next second output value. N can begreater than or equal to two and can be determined as one wishes todesign the system. As shown in the FIG. 4, the ½ rate convolutionalencoder receives u and outputs d₀ and d₁. d₀ and d₁ then become theoutput bits c₁ and c₂ of the TCM encoder. Therefore, when d₁d₀=00,c₂c₁=00, and the corresponding 8VSB symbol becomes 7 (c₂c₁c₀=000) or −5(c₂c₁c₀=001) depending on the value of c₀. c₀ is equal to the valuestored in a second memory s₁ and is obtained by adding s₀ and d₀, wheres₀ is the value stored in a first memory. The 8VSB symbols for d₁d₀=01,10, 11 are (−3, −1), (1,3), and (5,7), respectively.

FIG. 5A illustrates a non-systematic ½-rate convolutional encoder usedin a VSB system according to the present invention and its correspondingstate transition diagram. This type of encoder is often used because ofits long free-distance property. In the state transition diagram shownin FIG. 5A, a transition from the state S_(k) at t=k to the stateS_(k+1) at t=k+1 is denoted as a branch, and the value indicated aboveeach branch corresponds to the output of the branch. The probability ofreceiving a signal r when a signal z having zero mean and variance σ² issent through a AWGN channel can be obtained by using the equation:

$\begin{matrix}{{p\left( r \middle| z \right)} = {\frac{1}{\sqrt{2\pi\;\sigma^{2}}}{\exp\left( \frac{- \left| {r - z} \right|^{2}}{2\sigma^{2}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$where z represents a branch output. A branch metric is a probabilitymeasure of receiving r when the branch output z is sent from theencoder. It is an Euclidean distance between r and z, and can beobtained by the following equation:Branch Metric ∝Log(p(r/z))=|r−z| ².   [Equation 2]

A metric corresponding to a path including S₀, S₁, S₂, . . . , S_(k) canbe calculated by the equation:

$\begin{matrix}{{{Path}\mspace{14mu}{Metric}} = {\sum\limits_{t = 0}^{t = k}{{Branch}\mspace{14mu}{{Metric}.}}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$The path metric is an accumulated value of the branch metrics of thebranches included in a path and represents a probability of the path.

As shown in the state transition diagram of FIG. 5A, two branches aredivided from each S_(k), and two branches are merged into each S_(k+1).A viterbi decoder that decodes a convolutional code first calculates thepath metrics of the two paths that are merging into each state andselects the path having a lower path metric. The path metric selectedusing this technique represents the lowest path metric of the pathsstarting from an initial state (t=0) to each S_(k).

When selecting a path between two paths merging into one state, theprobability of the path selection becomes higher as the differencebetween the metrics of the two paths is larger. Since a path metricrepresents the sum of metrics of the branches included in a path, it isdesired to have the largest difference between the branch metrics inorder to maximize the performance of the encoder.

The ½ rate convolutional encoder shown in FIG. 5A includes a firstmemory for storing an input bit u as a first memory value s₀; a secondmemory for storing s₀ as a second memory value s₁; a first adder foradding u and s₁; and a second adder for adding u, s₀, and s₁. The outputfrom the first and second adders becomes a first output bit d₁ and asecond output bit d₀.

FIG. 5B illustrates a systematic convolutional encoder used in a VSBtransmission system and its corresponding state transition diagram. Afirst output bit d₁ is generated by bypassing an input bit u, and asecond output bit d₀ is generated by adding and delaying u. Thesystematic ½ rate convolutional encoder includes a first memory forstoring a previous second output bit value as a first memory value s₀,an adder for adding the input bit u and s₀, and a second memory forstoring the output from the adder as a second memory value s₁ andoutputting s₁ as the second output bit d₀.

According to FIG. 5A, the combination of the branch outputs dividingfrom a state at t=k or merging into a state at t=k+1 is (00,11) or(01,10). According to the trellis-coded modulation fundamental, theencoder has a better performance as the difference between branchmetrics of the combination is larger. A larger difference between thebranch metrics means that the corresponding Euclidean distance islarger. The Euclidean distance of (00,11) is larger than that of(01,10). When the output is either 01 or 10, the error often occursduring the path selection. Therefore, it is desired to have thecombination of the branch outputs of (00,10) and (01,11) so that thedifference between the branch metrics is large. This is shown in FIG.5B. Therefore, the convolutional encoder of FIG. 5B has a betterencoding performance than that of FIG. 5A.

FIG. 6 illustrates a VSB receiving system corresponding to the VSBtransmission system of the present invention.

FIG. 7A and FIG. 7B illustrate Euclidean distances corresponding to theoutput combinations generated from the encoders shown in FIG. 5A andFIG. 5B, respectively. As it can be shown from both figures, theEuclidean distances of (00,10) and (01,11) are much larger than the thatof (01,10). Therefore, the convolutional encoder of FIG. 5B has a betterperformance when connected to the ⅔ rate TCM encoder in the VSBtransmission system.

FIG. 8 illustrates performances of ATSC 8-VSB transmission systems wheneach of the convolutional encoders shown in FIG. 5A and FIG. 5B is usedin the system. For a bit error rate of 1e−3, the signal to noise ratiois reduced by 2 dB and 4 dB when the convolutional encoders shown inFIG. 5A and FIG. 5B are used as an additional error-correcting encoderin the VSB system. Therefore, a bit error rate can be reduced by using a½ rate convolutional encoder as an outer encoder of the TCM encoder, andthe encoder shown in FIG. 5B has a better bit error rate reductionproperty.

In conclusion, data can be transmitted at a lower signal to noise ratioby concatenating a ½ rate convolutional encoder to the TCM encoder in aVSB transmission system according the present invention.

The forgoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teachings canbe readily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims. Many alternatives, modifications, and variationswill be apparent to those skilled in the art.

1. A digital broadcast transmitter, comprising: a convolutional encoderencoding an input signal to generate first and second output signals;and a trellis-coded modulation (TCM) encoder encoding the first andsecond output signals to generate third, fourth, and fifth outputsignals, wherein the convolutional encoder comprises: a plurality ofmultipliers, each i th multiplier multiplying the input signal by aconstant k_(i) to generate an i th multiplier value, wherein theconstant k_(i) is greater than 1 for at least one value of i; aplurality of memories, a first memory storing a previous value of thesecond output signal as a first memory value and each i+1 th memorystoring an i+1 th memory value obtained by adding an i th memory valuestored in an i th memory and the i th multiplier value; and a pluralityof adders, each i th adder adding the i th memory value and the i thmultiplier value, wherein i=1, 2, 3, . . . , n, and the n+1 th memoryvalue stored in the n+1 th memory is a current value of the secondoutput signal.
 2. A method for encoding an input signal in a digitalbroadcast transmitter having a convolutional encoder and a trellis-codedmodulation (TCM) encoder, the method comprising the steps of: generatingfirst and second output signals by encoding the input signal using theconvolutional encoder; and generating third, fourth, and fifth outputsignals by encoding the first and second output signals using the TCMencoder, wherein a current value of the second output signal isgenerated by the steps of: multiplying the input signal by a constantk_(i) to generate an i th multiplier value for i=1, 2, 3 . . . n,wherein the constant k_(i) is greater than 1 for at least one value ofi; storing a previous value of the second output signal as a firstmemory value; and storing an i+1 th memory value obtained by adding an ith memory value and the i th multiplier value for i=1, 2, 3 . . . n,wherein the current value of the second output signal is the n +1 thmemory value obtained.
 3. A method of processing a digital television(DTV) signal in a DIV receiver, the method comprising: receiving digitalbroadcast data, wherein the digital broadcast data result from encodingan input bit using a convolutional encoder to output first and seconddata bits, multiplexing the first and second data bits with main databits using a multiplexer to output third and fourth data bits, encodingthe third and fourth data bits using a trellis encoder to output fifth,sixth, and seventh data bits, and mapping the fifth, sixth, and seventhdata bits using a VSB mapper to output a symbol; and performing trellisdecoding on the received broadcast data, wherein encoding the input bitusing a convolutional encoder comprises; pre-storing a first memoryvalue in a first memory, the first memory value being a previous seconddata bit output from the convolutional encoder; adding the input bit tothe first memory value pre-stored in the first memory to obtain a secondmemory value; and storing the second memory value in a second memory,wherein the first data bit is based on the input bit and the second databit is the second memory value stored in the second memory.
 4. Themethod of claim 3, wherein encoding the third and fourth data bits usinga trellis encoder comprises: pre-storing a third memory value in a thirdmemory, the third memory value being a previous seventh data bit outputfrom the trellis encoder; adding the fourth data bit to the third memoryvalue to obtain a fourth memory value; and storing the fourth memoryvalue in a fourth memory, wherein the fifth data bit is the third databit output from the multiplexer, the sixth data bit is the fourth databit output from the multiplexer, and the seventh data bit is the fourthmemory value stored in the fourth memory.
 5. The method of claim 3,further comprising additionally decoding the trellis-decoded broadcastdata.
 6. A digital television (DTV) receiver comprising: receiving meansfor receiving digital broadcast data, wherein the digital broadcast dataresult from encoding an input bit using a convolutional encoder tooutput first and second data bits, multiplexing the first and seconddata bits with main data bits using a multiplexer to output third andfourth data bits, encoding the third and fourth data bits using atrellis encoder to output fifth, sixth, and seventh data bits, andmapping the fifth, sixth, and seventh data bits using a VSB mapper tooutput a symbol; and a trellis decoder for performing trellis decodingon the received broadcast data, wherein encoding an input bit using aconvolutional encoder comprises: pre-storing a first memory value in afirst memory, the first memory value being a previous second data bitoutput from the convolutional encoder; adding the input bit to the firstmemory value pre-stored in the first memory to obtain a second memoryvalue; and storing the second memory value in a second memory, whereinthe first data bit is based on the input bit and the second data bit isthe second memory value stored in the second memory.
 7. The DIV receiverof claim 6, wherein encoding the third and fourth data bits using atrellis encoder comprises: pre-storing a third memory value in a thirdmemory, the third memory value being a previous seventh data bit outputfrom the trellis encoder; adding the fourth data bit to the third memoryvalue to obtain a fourth memory value; and storing the fourth memoryvalue in a fourth memory, wherein the fifth data bit is the third databit output from the multiplexer, the sixth data bit is the fourth databit output from the multiplexer, and the seventh data bit is the fourthmemory value stored in the fourth memory.
 8. The DIV receiver of claim6, further comprising a decoder for additionally decoding thetrellis-decoded broadcast data.